docs: add note that project folder must be a Git repository

[weisserj]

Summary

• Added a note to the Install section clarifying that deepwork install requires a Git repository
• Added the same note to the Quick Start section for consistency
• Suggests running git init first if the folder isn't already a repo

This addresses a common point of confusion for new users who try to run deepwork install in a non-Git folder and get an error.

Test plan

• Verified the README renders correctly with the new notes

[github-actions]

All contributors have signed the CLA. ✅
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[weisserj]

I have read the CLA Document and I hereby sign the CLA